Since Linux 2.6.23, the default scheduler is CFS, the "Completely
Fair Scheduler". The CFS scheduler replaced the earlier "O(1)"
scheduler.
API summary
Linux provides the following system calls for controlling the CPU
scheduling behavior, policy, and priority of processes (or, more
precisely, threads).
nice(2)
Set a new nice value for the calling thread, and return
the new nice value.
getpriority(2)
Return the nice value of a thread, a process group, or the
set of threads owned by a specified user.
setpriority(2)
Set the nice value of a thread, a process group, or the
set of threads owned by a specified user.
sched_setscheduler(2)
Set the scheduling policy and parameters of a specified
thread.
sched_getscheduler(2)
Return the scheduling policy of a specified thread.
sched_setparam(2)
Set the scheduling parameters of a specified thread.
sched_getparam(2)
Fetch the scheduling parameters of a specified thread.
sched_get_priority_max(2)
Return the maximum priority available in a specified
scheduling policy.
sched_get_priority_min(2)
Return the minimum priority available in a specified
scheduling policy.
sched_rr_get_interval(2)
Fetch the quantum used for threads that are scheduled
under the "round-robin" scheduling policy.
sched_yield(2)
Cause the caller to relinquish the CPU, so that some other
thread be executed.
sched_setaffinity(2)
(Linux-specific) Set the CPU affinity of a specified
thread.
sched_getaffinity(2)
(Linux-specific) Get the CPU affinity of a specified
thread.
sched_setattr(2)
Set the scheduling policy and parameters of a specified
thread. This (Linux-specific) system call provides a
superset of the functionality of sched_setscheduler(2) and
sched_setparam(2).
sched_getattr(2)
Fetch the scheduling policy and parameters of a specified
thread. This (Linux-specific) system call provides a
superset of the functionality of sched_getscheduler(2) and
sched_getparam(2).
Scheduling policies
The scheduler is the kernel component that decides which runnable
thread will be executed by the CPU next. Each thread has an
associated scheduling policy and a static scheduling priority,
sched_priority. The scheduler makes its decisions based on
knowledge of the scheduling policy and static priority of all
threads on the system.
For threads scheduled under one of the normal scheduling policies
(SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not
used in scheduling decisions (it must be specified as 0).
Processes scheduled under one of the real-time policies
(SCHED_FIFO, SCHED_RR) have a sched_priority value in the range 1
(low) to 99 (high). (As the numbers imply, real-time threads
always have higher priority than normal threads.) Note well:
POSIX.1 requires an implementation to support only a minimum 32
distinct priority levels for the real-time policies, and some
systems supply just this minimum. Portable programs should use
sched_get_priority_min(2) and sched_get_priority_max(2) to find
the range of priorities supported for a particular policy.
Conceptually, the scheduler maintains a list of runnable threads
for each possible sched_priority value. In order to determine
which thread runs next, the scheduler looks for the nonempty list
with the highest static priority and selects the thread at the
head of this list.
A thread's scheduling policy determines where it will be inserted
into the list of threads with equal static priority and how it
will move inside this list.
All scheduling is preemptive: if a thread with a higher static
priority becomes ready to run, the currently running thread will
be preempted and returned to the wait list for its static
priority level. The scheduling policy determines the ordering
only within the list of runnable threads with equal static
priority.
SCHED_FIFO: First in-first out scheduling
SCHED_FIFO can be used only with static priorities higher than 0,
which means that when a SCHED_FIFO thread becomes runnable, it
will always immediately preempt any currently running
SCHED_OTHER, SCHED_BATCH, or SCHED_IDLE thread. SCHED_FIFO is a
simple scheduling algorithm without time slicing. For threads
scheduled under the SCHED_FIFO policy, the following rules apply:
1) A running SCHED_FIFO thread that has been preempted by another
thread of higher priority will stay at the head of the list
for its priority and will resume execution as soon as all
threads of higher priority are blocked again.
2) When a blocked SCHED_FIFO thread becomes runnable, it will be
inserted at the end of the list for its priority.
3) If a call to sched_setscheduler(2), sched_setparam(2),
sched_setattr(2), pthread_setschedparam(3), or
pthread_setschedprio(3) changes the priority of the running or
runnable SCHED_FIFO thread identified by pid the effect on the
thread's position in the list depends on the direction of the
change to threads priority:
• If the thread's priority is raised, it is placed at the end
of the list for its new priority. As a consequence, it may
preempt a currently running thread with the same priority.
• If the thread's priority is unchanged, its position in the
run list is unchanged.
• If the thread's priority is lowered, it is placed at the
front of the list for its new priority.
According to POSIX.1-2008, changes to a thread's priority (or
policy) using any mechanism other than pthread_setschedprio(3)
should result in the thread being placed at the end of the
list for its priority.
4) A thread calling sched_yield(2) will be put at the end of the
list.
No other events will move a thread scheduled under the SCHED_FIFO
policy in the wait list of runnable threads with equal static
priority.
A SCHED_FIFO thread runs until either it is blocked by an I/O
request, it is preempted by a higher priority thread, or it calls
sched_yield(2).
SCHED_RR: Round-robin scheduling
SCHED_RR is a simple enhancement of SCHED_FIFO. Everything
described above for SCHED_FIFO also applies to SCHED_RR, except
that each thread is allowed to run only for a maximum time
quantum. If a SCHED_RR thread has been running for a time period
equal to or longer than the time quantum, it will be put at the
end of the list for its priority. A SCHED_RR thread that has
been preempted by a higher priority thread and subsequently
resumes execution as a running thread will complete the unexpired
portion of its round-robin time quantum. The length of the time
quantum can be retrieved using sched_rr_get_interval(2).
SCHED_DEADLINE: Sporadic task model deadline scheduling
Since version 3.14, Linux provides a deadline scheduling policy
(SCHED_DEADLINE). This policy is currently implemented using
GEDF (Global Earliest Deadline First) in conjunction with CBS
(Constant Bandwidth Server). To set and fetch this policy and
associated attributes, one must use the Linux-specific
sched_setattr(2) and sched_getattr(2) system calls.
A sporadic task is one that has a sequence of jobs, where each
job is activated at most once per period. Each job also has a
relative deadline, before which it should finish execution, and a
computation time, which is the CPU time necessary for executing
the job. The moment when a task wakes up because a new job has
to be executed is called the arrival time (also referred to as
the request time or release time). The start time is the time at
which a task starts its execution. The absolute deadline is thus
obtained by adding the relative deadline to the arrival time.
The following diagram clarifies these terms:
arrival/wakeup absolute deadline
| start time |
| | |
v v v
-----x--------xooooooooooooooooo--------x--------x---
|<- comp. time ->|
|<------- relative deadline ------>|
|<-------------- period ------------------->|
When setting a SCHED_DEADLINE policy for a thread using
sched_setattr(2), one can specify three parameters: Runtime,
Deadline, and Period. These parameters do not necessarily
correspond to the aforementioned terms: usual practice is to set
Runtime to something bigger than the average computation time (or
worst-case execution time for hard real-time tasks), Deadline to
the relative deadline, and Period to the period of the task.
Thus, for SCHED_DEADLINE scheduling, we have:
arrival/wakeup absolute deadline
| start time |
| | |
v v v
-----x--------xooooooooooooooooo--------x--------x---
|<-- Runtime ------->|
|<----------- Deadline ----------->|
|<-------------- Period ------------------->|
The three deadline-scheduling parameters correspond to the
sched_runtime, sched_deadline, and sched_period fields of the
sched_attr structure; see sched_setattr(2). These fields express
values in nanoseconds. If sched_period is specified as 0, then
it is made the same as sched_deadline.
The kernel requires that:
sched_runtime <= sched_deadline <= sched_period
In addition, under the current implementation, all of the
parameter values must be at least 1024 (i.e., just over one
microsecond, which is the resolution of the implementation), and
less than 2^63. If any of these checks fails, sched_setattr(2)
fails with the error EINVAL.
The CBS guarantees non-interference between tasks, by throttling
threads that attempt to over-run their specified Runtime.
To ensure deadline scheduling guarantees, the kernel must prevent
situations where the set of SCHED_DEADLINE threads is not
feasible (schedulable) within the given constraints. The kernel
thus performs an admittance test when setting or changing
SCHED_DEADLINE policy and attributes. This admission test
calculates whether the change is feasible; if it is not,
sched_setattr(2) fails with the error EBUSY.
For example, it is required (but not necessarily sufficient) for
the total utilization to be less than or equal to the total
number of CPUs available, where, since each thread can maximally
run for Runtime per Period, that thread's utilization is its
Runtime divided by its Period.
In order to fulfill the guarantees that are made when a thread is
admitted to the SCHED_DEADLINE policy, SCHED_DEADLINE threads are
the highest priority (user controllable) threads in the system;
if any SCHED_DEADLINE thread is runnable, it will preempt any
thread scheduled under one of the other policies.
A call to fork(2) by a thread scheduled under the SCHED_DEADLINE
policy fails with the error EAGAIN, unless the thread has its
reset-on-fork flag set (see below).
A SCHED_DEADLINE thread that calls sched_yield(2) will yield the
current job and wait for a new period to begin.
SCHED_OTHER: Default Linux time-sharing scheduling
SCHED_OTHER can be used at only static priority 0 (i.e., threads
under real-time policies always have priority over SCHED_OTHER
processes). SCHED_OTHER is the standard Linux time-sharing
scheduler that is intended for all threads that do not require
the special real-time mechanisms.
The thread to run is chosen from the static priority 0 list based
on a dynamic priority that is determined only inside this list.
The dynamic priority is based on the nice value (see below) and
is increased for each time quantum the thread is ready to run,
but denied to run by the scheduler. This ensures fair progress
among all SCHED_OTHER threads.
In the Linux kernel source code, the SCHED_OTHER policy is
actually named SCHED_NORMAL.
The nice value
The nice value is an attribute that can be used to influence the
CPU scheduler to favor or disfavor a process in scheduling
decisions. It affects the scheduling of SCHED_OTHER and
SCHED_BATCH (see below) processes. The nice value can be
modified using nice(2), setpriority(2), or sched_setattr(2).
According to POSIX.1, the nice value is a per-process attribute;
that is, the threads in a process should share a nice value.
However, on Linux, the nice value is a per-thread attribute:
different threads in the same process may have different nice
values.
The range of the nice value varies across UNIX systems. On
modern Linux, the range is -20 (high priority) to +19 (low
priority). On some other systems, the range is -20..20. Very
early Linux kernels (Before Linux 2.0) had the range
-infinity..15.
The degree to which the nice value affects the relative
scheduling of SCHED_OTHER processes likewise varies across UNIX
systems and across Linux kernel versions.
With the advent of the CFS scheduler in kernel 2.6.23, Linux
adopted an algorithm that causes relative differences in nice
values to have a much stronger effect. In the current
implementation, each unit of difference in the nice values of two
processes results in a factor of 1.25 in the degree to which the
scheduler favors the higher priority process. This causes very
low nice values (+19) to truly provide little CPU to a process
whenever there is any other higher priority load on the system,
and makes high nice values (-20) deliver most of the CPU to
applications that require it (e.g., some audio applications).
On Linux, the RLIMIT_NICE resource limit can be used to define a
limit to which an unprivileged process's nice value can be
raised; see setrlimit(2) for details.
For further details on the nice value, see the subsections on the
autogroup feature and group scheduling, below.
SCHED_BATCH: Scheduling batch processes
(Since Linux 2.6.16.) SCHED_BATCH can be used only at static
priority 0. This policy is similar to SCHED_OTHER in that it
schedules the thread according to its dynamic priority (based on
the nice value). The difference is that this policy will cause
the scheduler to always assume that the thread is CPU-intensive.
Consequently, the scheduler will apply a small scheduling penalty
with respect to wakeup behavior, so that this thread is mildly
disfavored in scheduling decisions.
This policy is useful for workloads that are noninteractive, but
do not want to lower their nice value, and for workloads that
want a deterministic scheduling policy without interactivity
causing extra preemptions (between the workload's tasks).
SCHED_IDLE: Scheduling very low priority jobs
(Since Linux 2.6.23.) SCHED_IDLE can be used only at static
priority 0; the process nice value has no influence for this
policy.
This policy is intended for running jobs at extremely low
priority (lower even than a +19 nice value with the SCHED_OTHER
or SCHED_BATCH policies).
Resetting scheduling policy for child processes
Each thread has a reset-on-fork scheduling flag. When this flag
is set, children created by fork(2) do not inherit privileged
scheduling policies. The reset-on-fork flag can be set by
either:
* ORing the SCHED_RESET_ON_FORK flag into the policy argument
when calling sched_setscheduler(2) (since Linux 2.6.32); or
* specifying the SCHED_FLAG_RESET_ON_FORK flag in
attr.sched_flags when calling sched_setattr(2).
Note that the constants used with these two APIs have different
names. The state of the reset-on-fork flag can analogously be
retrieved using sched_getscheduler(2) and sched_getattr(2).
The reset-on-fork feature is intended for media-playback
applications, and can be used to prevent applications evading the
RLIMIT_RTTIME resource limit (see getrlimit(2)) by creating
multiple child processes.
More precisely, if the reset-on-fork flag is set, the following
rules apply for subsequently created children:
* If the calling thread has a scheduling policy of SCHED_FIFO or
SCHED_RR, the policy is reset to SCHED_OTHER in child
processes.
* If the calling process has a negative nice value, the nice
value is reset to zero in child processes.
After the reset-on-fork flag has been enabled, it can be reset
only if the thread has the CAP_SYS_NICE capability. This flag is
disabled in child processes created by fork(2).
Privileges and resource limits
In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE)
threads can set a nonzero static priority (i.e., set a real-time
scheduling policy). The only change that an unprivileged thread
can make is to set the SCHED_OTHER policy, and this can be done
only if the effective user ID of the caller matches the real or
effective user ID of the target thread (i.e., the thread
specified by pid) whose policy is being changed.
A thread must be privileged (CAP_SYS_NICE) in order to set or
modify a SCHED_DEADLINE policy.
Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a
ceiling on an unprivileged thread's static priority for the
SCHED_RR and SCHED_FIFO policies. The rules for changing
scheduling policy and priority are as follows:
* If an unprivileged thread has a nonzero RLIMIT_RTPRIO soft
limit, then it can change its scheduling policy and priority,
subject to the restriction that the priority cannot be set to
a value higher than the maximum of its current priority and
its RLIMIT_RTPRIO soft limit.
* If the RLIMIT_RTPRIO soft limit is 0, then the only permitted
changes are to lower the priority, or to switch to a non-real-
time policy.
* Subject to the same rules, another unprivileged thread can
also make these changes, as long as the effective user ID of
the thread making the change matches the real or effective
user ID of the target thread.
* Special rules apply for the SCHED_IDLE policy. In Linux
kernels before 2.6.39, an unprivileged thread operating under
this policy cannot change its policy, regardless of the value
of its RLIMIT_RTPRIO resource limit. In Linux kernels since
2.6.39, an unprivileged thread can switch to either the
SCHED_BATCH or the SCHED_OTHER policy so long as its nice
value falls within the range permitted by its RLIMIT_NICE
resource limit (see getrlimit(2)).
Privileged (CAP_SYS_NICE) threads ignore the RLIMIT_RTPRIO limit;
as with older kernels, they can make arbitrary changes to
scheduling policy and priority. See getrlimit(2) for further
information on RLIMIT_RTPRIO.
Limiting the CPU usage of real-time and deadline processes
A nonblocking infinite loop in a thread scheduled under the
SCHED_FIFO, SCHED_RR, or SCHED_DEADLINE policy can potentially
block all other threads from accessing the CPU forever. Prior to
Linux 2.6.25, the only way of preventing a runaway real-time
process from freezing the system was to run (at the console) a
shell scheduled under a higher static priority than the tested
application. This allows an emergency kill of tested real-time
applications that do not block or terminate as expected.
Since Linux 2.6.25, there are other techniques for dealing with
runaway real-time and deadline processes. One of these is to use
the RLIMIT_RTTIME resource limit to set a ceiling on the CPU time
that a real-time process may consume. See getrlimit(2) for
details.
Since version 2.6.25, Linux also provides two /proc files that
can be used to reserve a certain amount of CPU time to be used by
non-real-time processes. Reserving CPU time in this fashion
allows some CPU time to be allocated to (say) a root shell that
can be used to kill a runaway process. Both of these files
specify time values in microseconds:
/proc/sys/kernel/sched_rt_period_us
This file specifies a scheduling period that is equivalent
to 100% CPU bandwidth. The value in this file can range
from 1 to INT_MAX, giving an operating range of 1
microsecond to around 35 minutes. The default value in
this file is 1,000,000 (1 second).
/proc/sys/kernel/sched_rt_runtime_us
The value in this file specifies how much of the "period"
time can be used by all real-time and deadline scheduled
processes on the system. The value in this file can range
from -1 to INT_MAX-1. Specifying -1 makes the run time
the same as the period; that is, no CPU time is set aside
for non-real-time processes (which was the Linux behavior
before kernel 2.6.25). The default value in this file is
950,000 (0.95 seconds), meaning that 5% of the CPU time is
reserved for processes that don't run under a real-time or
deadline scheduling policy.
Response time
A blocked high priority thread waiting for I/O has a certain
response time before it is scheduled again. The device driver
writer can greatly reduce this response time by using a "slow
interrupt" interrupt handler.
Miscellaneous
Child processes inherit the scheduling policy and parameters
across a fork(2). The scheduling policy and parameters are
preserved across execve(2).
Memory locking is usually needed for real-time processes to avoid
paging delays; this can be done with mlock(2) or mlockall(2).
The autogroup feature
Since Linux 2.6.38, the kernel provides a feature known as
autogrouping to improve interactive desktop performance in the
face of multiprocess, CPU-intensive workloads such as building
the Linux kernel with large numbers of parallel build processes
(i.e., the make(1) -j flag).
This feature operates in conjunction with the CFS scheduler and
requires a kernel that is configured with CONFIG_SCHED_AUTOGROUP.
On a running system, this feature is enabled or disabled via the
file /proc/sys/kernel/sched_autogroup_enabled; a value of 0
disables the feature, while a value of 1 enables it. The default
value in this file is 1, unless the kernel was booted with the
noautogroup parameter.
A new autogroup is created when a new session is created via
setsid(2); this happens, for example, when a new terminal window
is started. A new process created by fork(2) inherits its
parent's autogroup membership. Thus, all of the processes in a
session are members of the same autogroup. An autogroup is
automatically destroyed when the last process in the group
terminates.
When autogrouping is enabled, all of the members of an autogroup
are placed in the same kernel scheduler "task group". The CFS
scheduler employs an algorithm that equalizes the distribution of
CPU cycles across task groups. The benefits of this for
interactive desktop performance can be described via the
following example.
Suppose that there are two autogroups competing for the same CPU
(i.e., presume either a single CPU system or the use of
taskset(1) to confine all the processes to the same CPU on an SMP
system). The first group contains ten CPU-bound processes from a
kernel build started with make -j10. The other contains a single
CPU-bound process: a video player. The effect of autogrouping is
that the two groups will each receive half of the CPU cycles.
That is, the video player will receive 50% of the CPU cycles,
rather than just 9% of the cycles, which would likely lead to
degraded video playback. The situation on an SMP system is more
complex, but the general effect is the same: the scheduler
distributes CPU cycles across task groups such that an autogroup
that contains a large number of CPU-bound processes does not end
up hogging CPU cycles at the expense of the other jobs on the
system.
A process's autogroup (task group) membership can be viewed via
the file /proc/[pid]/autogroup:
$ cat /proc/1/autogroup
/autogroup-1 nice 0
This file can also be used to modify the CPU bandwidth allocated
to an autogroup. This is done by writing a number in the "nice"
range to the file to set the autogroup's nice value. The allowed
range is from +19 (low priority) to -20 (high priority).
(Writing values outside of this range causes write(2) to fail
with the error EINVAL.)
The autogroup nice setting has the same meaning as the process
nice value, but applies to distribution of CPU cycles to the
autogroup as a whole, based on the relative nice values of other
autogroups. For a process inside an autogroup, the CPU cycles
that it receives will be a product of the autogroup's nice value
(compared to other autogroups) and the process's nice value
(compared to other processes in the same autogroup.
The use of the cgroups(7) CPU controller to place processes in
cgroups other than the root CPU cgroup overrides the effect of
autogrouping.
The autogroup feature groups only processes scheduled under non-
real-time policies (SCHED_OTHER, SCHED_BATCH, and SCHED_IDLE).
It does not group processes scheduled under real-time and
deadline policies. Those processes are scheduled according to
the rules described earlier.
The nice value and group scheduling
When scheduling non-real-time processes (i.e., those scheduled
under the SCHED_OTHER, SCHED_BATCH, and SCHED_IDLE policies), the
CFS scheduler employs a technique known as "group scheduling", if
the kernel was configured with the CONFIG_FAIR_GROUP_SCHED option
(which is typical).
Under group scheduling, threads are scheduled in "task groups".
Task groups have a hierarchical relationship, rooted under the
initial task group on the system, known as the "root task group".
Task groups are formed in the following circumstances:
* All of the threads in a CPU cgroup form a task group. The
parent of this task group is the task group of the
corresponding parent cgroup.
* If autogrouping is enabled, then all of the threads that are
(implicitly) placed in an autogroup (i.e., the same session,
as created by setsid(2)) form a task group. Each new
autogroup is thus a separate task group. The root task group
is the parent of all such autogroups.
* If autogrouping is enabled, then the root task group consists
of all processes in the root CPU cgroup that were not
otherwise implicitly placed into a new autogroup.
* If autogrouping is disabled, then the root task group consists
of all processes in the root CPU cgroup.
* If group scheduling was disabled (i.e., the kernel was
configured without CONFIG_FAIR_GROUP_SCHED), then all of the
processes on the system are notionally placed in a single task
group.
Under group scheduling, a thread's nice value has an effect for
scheduling decisions only relative to other threads in the same
task group. This has some surprising consequences in terms of
the traditional semantics of the nice value on UNIX systems. In
particular, if autogrouping is enabled (which is the default in
various distributions), then employing setpriority(2) or nice(1)
on a process has an effect only for scheduling relative to other
processes executed in the same session (typically: the same
terminal window).
Conversely, for two processes that are (for example) the sole
CPU-bound processes in different sessions (e.g., different
terminal windows, each of whose jobs are tied to different
autogroups), modifying the nice value of the process in one of
the sessions has no effect in terms of the scheduler's decisions
relative to the process in the other session. A possibly useful
workaround here is to use a command such as the following to
modify the autogroup nice value for all of the processes in a
terminal session:
$ echo 10 > /proc/self/autogroup
Real-time features in the mainline Linux kernel
Since kernel version 2.6.18, Linux is gradually becoming equipped
with real-time capabilities, most of which are derived from the
former realtime-preempt patch set. Until the patches have been
completely merged into the mainline kernel, they must be
installed to achieve the best real-time performance. These
patches are named:
patch-kernelversion-rtpatchversion
and can be downloaded from
⟨http://www.kernel.org/pub/linux/kernel/projects/rt/⟩.
Without the patches and prior to their full inclusion into the
mainline kernel, the kernel configuration offers only the three
preemption classes CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY,
and CONFIG_PREEMPT_DESKTOP which respectively provide no, some,
and considerable reduction of the worst-case scheduling latency.
With the patches applied or after their full inclusion into the
mainline kernel, the additional configuration item
CONFIG_PREEMPT_RT becomes available. If this is selected, Linux
is transformed into a regular real-time operating system. The
FIFO and RR scheduling policies are then used to run a thread
with true real-time priority and a minimum worst-case scheduling
latency.
